[crossfire] Show invisible behaviour

[Nicolas Weeger (Laposte)]

Hello.

With relation to bug 
https://sourceforge.net/tracker/index.php?func=detail&aid=1556723&group_id=13833&atid=113833 
about broken show invisible, i've changed the behaviour for that spell:

chance to show a hidden item is now (casting level-1) on (item's level).

This just means monsters over 110 will never be shown :)

If anyone has another idea, feel free to explain it ;)

Nicolas